The present invention relates to an arrangement and a method for reducing the nitrogen oxide content in the exhaust gas of an internal combustion engine with the aid of ammonia and/or ammonia-releasing reduction agents.
Nitrogen oxides belong to the limited exhaust gas components that are produced during combustion processes and the permissible emissions of which are continuously being lowered. In this connection, the reduction of the nitrogen oxides generally occurs with the aid of catalysts. In oxygen-rich exhaust gas, a reduction agent is additionally necessary in order to raise the selectivity and the NOx conversions. These methods have become known under the general term SCR processes, whereby SCR stands for “Selective Catalytic Reduction”. They have been used for many years in the power plant industry, and in recent times also with internal combustion engines. A detailed description of such methods can be found in DE 34 28 232 A1. V2O5- containing mixed oxides, for example in the form of V2O5/WO3/TiO2, can be used as SCR catalysts. Typical of V2O5 percentages are between 0.2-3%. Ammonia or ammonia-releasing compounds, such as urea or ammonium formate in solid or dissolved form, are used in practical applications. For the conversion of one mole of nitric oxide, one mole of ammonia is necessary.4NO+4NH3+O24N2+6H2O  (1)
If a platinum-containing NO oxidation catalyst is disposed upstream of the SCR catalyst to form NO2,2NO+O22NO2  (2)
the SCR reaction can be significantly accelerated and the low temperature activity can be considerably raised.
The SCR process causes special problems with the reduction of nitrogen oxides of internal combustion engines, and here in particular in vehicles, since care must be taken that there is no emission of unused ammonia. In contrast to the situation with power plants, in vehicles, no adequately precise and stable exhaust gas sensors are available for regulating the system and hence for avoiding NH3 emissions when an overdosing of reduction agent occurs. In addition, the use of V2O5 is problematic since at temperatures over 650° C. it sublimes, so that in recent times zeolite having the active substituents iron and/or copper and/or cobalt have been used.
In order despite the inadequate sensor technology to avoid undesired NH3 emissions, without additional measures the SCR catalyst must be significantly over dimensioned in order to ensure adequate reliability against ammonia slippage. The situation can be improved if an NH3-oxidation catalyst is provided downstream of the SCR catalyst. Such an arrangement is shown, for example, in DE 37 33 501 A1. Furthermore, it is known from EP 410 440 B1 to provide the SCR catalyst and the NH3-oxidation catalyst on a common support or substrate.
Noble metals of the platinum group, as well as oxides thereof, can be used as the active material for the NH3-oxidation catalyst. The intended task of the NH3-oxidation catalyst of oxidizing excess NH3 to nitrogen can be realized only inadequately in practice due to the too low selectivity of the active substituents, for example platinum-containing substituents, so that the oxidation, as shown in the following equations, rather than ending at the oxidation state [0] ends at the oxidation states [+1], [+2] or even only at [+4] and thus again nitrogen oxides result.4NH3+3O22N2+6H2O[0]  (3)2NH3+2.5O22NO+3H2O[+2]  (4)2NO+O22NO2[+4]  (5)NH3+NO22NO+H2O[+2]  (6)In addition, the platinum metals (platinum, palladium, rhodium, iridium, osmium, ruthenium) as well as their oxides, that are used as active material for the NH3-oxidation catalyst, are very expensive and rare. For this reason , the NH3-oxidation catalysts are generally made very small, which means that they are often overloaded, so that there is no complete NH3 conversion.
It is an object of the present invention to provide an arrangement that on the one hand reliably presents ammonia slippage, and on the other hand reduces the residual nitrogen oxide content in the exhaust gas to a minimum. It is furthermore an object to provide a method for producing such an arrangement.